1. Field of the Invention
The present invention relates generally to antennas and more particularly to phased array antennas.
2. Description of the Related Art
A portion of a conventional multibeam phased array antenna 20 is shown in FIG. 1. The antenna has a plurality of radiators 22 that are arranged along an array face 24. The radiator array is typically divided into subarrays. For example, the array might contain 1024 radiators that are divided into four subarrays that each contain 256 radiators. For simplicity, FIG. 1 illustrates a single 16 element row in one of these subarrays.
In each row, each radiator 22 is coupled by a power amplifier 28 to a respective multiplexer 30. Each radiated beam is associated with a different manifold 32 that has a primary transmission line 34 which branches into secondary transmission lines 36 that each couple to a respective one of the multiplexers 30. A programmable delay line 38 is inserted into the primary transmission line 34 and a filter 40 and an adjustable electrical phase shifter 42 are inserted into each secondary transmission line 36. For clarity of illustration, each primary transmission line is labeled with the number of its respective antenna beam.
Operation of the phased array antenna can be separated into coarse and fine beam pointing processes. In a coarse beam pointing process, an appropriate time delay is programmed into each beam #1 delay line of the four subarrays. These time delays generate a selected coarse phase front (e.g., the coarse phase front 44) across the antenna array and, accordingly, a #1 antenna beam is radiated orthogonally to that coarse phase front. In a fine beam pointing process, appropriate phase shifts are selected with the phase shifters 42 that are associated with the manifold of beam #1. These phase shifts modify the coarse phase front to generate a fine phase front (e.g., the fine phase front 46) across the antenna array and, accordingly, the #1 antenna beam is radiated orthogonally to that phase front. This operational process is repeated for each of the other beams, i.e., beams #2, #3 and #4.
When data (e.g., pulses) are placed on the radiated signals, the signal spectrum is widened. This can lead to an undesirable increase in beam divergence unless the phase shift .DELTA..phi. between adjacent antenna elements satisfies the relation .DELTA..phi.=2.pi.f.DELTA.t for all frequency components f with the signal spectrum. Here .DELTA.t is the time required for radiation to travel, for example, along the path 47 in FIG. 1 to form phase front 44. This undesirable beam broadening in wide bandwidth signals is commonly referred to as "beam squint".
In the antenna 20 of FIG. 1, the delay lines 38 insert an appropriate time delay .DELTA.t to form the coarse wavefront 44. This .DELTA.t is constant for all frequency components so that the adjacent-element phase shift satisfies the relation .DELTA..phi.=2.pi.f.DELTA.t. In contrast, the phase shifters 42 typically insert a phase shift that is approximately constant across the instantaneous signal bandwidth and thus the adjacent phases do not satisfy this relation. Accordingly, each radiated beam is preferably coarsely steered to a nominal beam angle and then finely steered about this nominal angle. The coarse steering will not induce beam squint but the fine steering will.
The spacing (49 in FIG. 1) between the radiators 22 is set so as to avoid the generation of undesirable beam lobes (typically called "grating lobes"). In an antenna in which the beam is to be steered .+-.90.degree., for example, the radiator spacing must be .ltoreq..lambda..sub.min .sup./2 in which .lambda..sub.min is the smallest radiated wavelength. If the antenna 20 of FIG. 1 is configured to operate over the bandwidth of 6 to 18 GHz, for example, the spacing might be on the order of 8.3 millimeters.
It can be appreciated, therefore, that it is a challenge to position multiple sets of phase shifters and filters into each radiator position along with an associated multiplexer and power amplifier. Each phase shifter also typically has associated control lines and may have an associated amplifier driver. These elements must also be fitted into a volume that is set by the radiator spacing 48. In addition, the electrical phase shifters 42 are typically lossy, complex and expensive. It would, therefore, be advantageous to have phased array structures that eliminate the need at the array face for multiple sets of phase shifters and their attendant multiplexers, driver amplifiers and control wires. Preferably, these structures should generate antenna beams that have low values of beam squint.